1. Field of the Invention
The present invention relates to a control system for an internal combustion engine, and particularly to a control system for an internal combustion engine which has a plurality of cylinders consisting of two or more cylinder groups, and a plurality of intake systems respectively corresponding to the cylinder groups.
2. Description of the Related Art
The brake booster for assisting the driver in operating the brake pedal of the automobile is widely used. The brake booster uses a negative pressure (a pressure which is lower than the atmospheric pressure) generated in the intake system of the internal combustion engine, to generate the assisting force. Japanese Patent Laid-open No. 2000-120474 (JP-'474) discloses a control system for an internal combustion engine which has two cylinder groups and two intake systems corresponding to the two cylinder groups. The engine and the brake booster shown in JP-'474 are configured so that the negative pressure is supplied from the two intake systems to the brake booster.
This control system is proposed contemplating a tendency that the negative pressure in the brake booster becomes insufficient when performing the lean-burn operation of the engine. According to this control system, the partition valve provided in the passage connecting the two intake systems, is opened when the negative pressure in the brake booster is sufficient. On the other hand, when the negative pressure is insufficient, the partition valve is closed and the throttle valve in the first intake system is controlled to be slightly closed while the throttle valve in the second intake system is controlled to an opening which is suitable for the lean-burn operation. Consequently, the negative pressure is secured by the first intake system.
When the brake pedal is operated and the vehicle is decreasing, an output of the engine for driving the vehicle may sometimes be increased for securing running stability of the vehicle. When the vehicle speed is decreasing, the throttle valve is normally fully-closed to secure a sufficient negative pressure. However, if the engine output is increased upon deceleration of the vehicle as described above, the negative pressure may become insufficient. Further, when performing the shift-down of the transmission upon vehicle deceleration, the blipping control wherein the engine output is temporarily increased so as to increase the engine rotational speed, is performed. Such blipping control is also one of the factors which make the negative pressure insufficient.
The control system shown in JP-'474 is configured for securing the negative pressure when performing the lean-burn operation of the engine. Therefore, the problem due to the engine output increase control upon vehicle deceleration cannot be solved by the control system of JP-'474.